1. Field of the Invention
The present invention relates generally to the measurement of ionizing radiation through the use of an ionization chamber. More particularly, the invention concerns a novel apparatus for determining the characteristics of a particular ionization chamber by measuring the capacitance of the chamber and using that capacitance to identify the particular type of chamber being used to measure the ionizing radiation.
2. Discussion of the Invention
In the simplest form, an ion chamber is merely a closed chamber containing an ionizing gas. Ionizing radiation, such as X-rays, gamma rays or charged particles interact with the gas in the ion chamber to produce ions. Typically, the ion chamber is connected to a read-out unit containing a bias supply which is used to establish an electric field within the ion chamber. The ions produced within the chamber are collected by the electric field and flow in an external circuit which typically comprises the bias supply and an electrometer. The electrometer functions to convert the small current within the ion chamber into an electrical signal that can be processed into values suitable for display by a display means such as a controller. As a general rule, this conversion includes allowance for sensitivity of the ion chamber and the electrometer. Calibration and identification information is used by the read-out to convert the electrical signal from the electrometer into units of radiation such as Roentgens. The Roentgen is generally defined in coulombs per unit volume of air at standard temperature and pressure.
Prior art ion chambers are of various sizes, shapes and sensitivities. For example, small volume ion chambers are typically used to measure intense radiation, while larger volume chambers are used to measure low level radiation. In like manner, very compact chambers are useful for measuring radiation with a small spacial extent, while paddle shaped chambers are often used for measuring radiation in a specific area. In both cases, parallel plane and cylindrical geometries can be employed.
A wide variety of materials are used in the construction of prior art ion chambers. For example, thin-window chambers are needed to measure low-energy radiation while thicker wall chambers may be required for higher energy measurements. While not all prior art ion chambers are gas filled, the sensitivity of gas filled ion chambers depends directly on the amount of gas within the chamber. In this regard, the sensitivity of prior art ion chambers varies substantially and volumes of prior art ion chambers can range between 0.1 cubic centimeters and 10 liters.
It is fundamental that in order for the read-out unit of the radiation measuring apparatus to accurately display the results of a measurement in radiation units, the sensitivity of the particular ion chamber to which the read-out unit is connected must be determined and appropriately entered. Frequently, this is done by the operator manually entering the sensitivity of the particular ion chamber being used in coulombs per Roentgen. Entering the sensitivity in this manner is both time consuming and prone to error. Another method sometimes used to enter the chamber sensitivity into the read-out unit is through the use of a coding means in the form of a wired connector or a coding resister which forms a part of the read-out itself. This approach lessens the time required to enter the sensitivity data, but involves a more complex electrical connection and requires additional circuitry to measure the coding.
The thrust of the present invention is to overcome the drawbacks of prior art radiation measuring apparatus as described in the preceding paragraph. More particularly, the present invention provides a novel means for identifying the specific type of ion chamber involved by first measuring the charge produced as the bias voltage on the chamber is varied in a known way and then using this information to identify the chamber type. This vastly simplifies the ion chamber by completely eliminating the need for coding wires or components. This, in turn, simplifies the design of the read-out unit since the unit does not have to respond to any coding arrangements.